Microwave signal generator output system design - Hewlett-Packard's HP 8665A Synthesized Signal Generator - includes related article on packageless microcircuits

Hewlett-Packard Journal, Oct, 1989 by Steve R. Fired, Keith L. Fries, John M. Sims

Microwave Signal Generator Output System Design

THE OUTPUT SYSTEM OF THE HP 8665A Synthesized Signal Generator takes the synthesized and divided signals from the GaAs divider described in the preceding article and produces an output signal in the range of 0.1 MHz to 4.2 GHz. It also provides automatic level control (ALC), amplitude modulation, and reverse power protection.

The main output section, which provides output frequencies from 0.1 MHz to 3 GHz, consists of a divided output section for frequencies from 0.1875 to 3GHz and a heterodyne output section for frequencies from 0.1 to 187.5 MHz. There is a separate microwave extender output section for frequencies from 3 to 4.2 GHz. Fig. 1 is a simplified block diagram of the HP 8665A output system.

Divided Output Section

The divided output section takes two of the octaves generated by the synthesis block and provides level control and AM capability for output frequencies from 187.5 MHz to 3.0 GHz. The upper three octaves of this range are covered by the high-frequency driver and the last octave is covered by the low-frequency driver, along with the heterodyne band, 0.1 to 187.5 MHz. The two drivers supply the inputs for the two sections of the main output amplifier.

High-Frequency Driver. The HF driver accepts two signals from the GaAs divider: 1.5 to 3.0 GHz (DIV2) and 0.75 to 1.5 GHz (DIV4). These signals have high harmonic content and their output levels vary with frequency. The HF driver filters the harmonics and levels the signals to provide a constant level to the high-band modulator. There is also a silicon binary divider on the HF driver which generates the 0.375-to-0.750-GHz octave. The block diagram of the HF driver is shown in Fig. 2. Because of the frequencies involved, the majority of the RF signal handling is done with microcircuits.

The desired frequency octave is selected by the input multiplexer on the premodulator microcircuit. If the 0.375-to-0.75-GHz band is selected, the DIV4 signal from the GaAs dividers is directed to the DIV8 silicon divider. The DIV8 output then returns to the premodulator microcircuit for further processing.

The next circuit on the microcircuit, the premodulator, acts like a variable attenuator controlled by the preleveling loop. If the drive to the premodulator increases by 2 dB then the premodulator attenuation will increase by 2 dB to keep the output level constant. Similarly, if a following stage rolls off by 1 dB the premodulator attenuation will decrease by 1 dB to ensure that the level at the input of the high-band modulator remains constant. While the premodulator performs a function similar to that of the high-band modulator, the performance requirements are much lower, so the design is simpler. The premodulator does not have to do amplitude modulation, so the dynamic range require is smaller. Also, harmonics are not very critical since the filter microcircuit follows the premodulator.

The premodulator circuit is followed by a two-stage GaAs amplifier. The lowest signal level in the HF driver signal path occurs at the input of this GaAs amplifier, so the noise floor is set at this point. Care must be taken to ensure that the instrument noise floor is not degraded by the HF driver. This is accomplished by carefully selecting the gain and flatness of the following stages, and by designing the amplifier for a low noise figure. Harmonics are not a major concern because the filter microcircuit attenuates them.

The filter microcircuit reduces the signal harmonics to the point where the harmonics at the output of the instrument are not dominated by the HF driver. The microcircuit contains two lumped-element low-pass filters and four transmission line low-pass filters. At high frequencies the transmission line filters no longer provide attenuation. This occurs at frequencies where the length of the filter elements is approximately one half wavelength. The lower the filter corner frequency, and therefore the larger the filter elements, the lower the frequency at which the filter no longer attenuates. To ensure that the filter microcircuit rejects very high-frequency harmonics, the 3-GHz filter is always left in the signal path. Being the highest frequency filter on the microcircuit, it provides isolation to the highest frequencies. An additional output is included to provide a 1000.1-MHz-to-1187.5-MHz signal to the down-converter section.

Following the filter is the high-band modulator microcircuit. This microcircuit includes the peak detector for the preleveling loop and the AM modulator for the 0.375-to-3.0-GHz band. Placing the preleving loop peak detector inside this microcircuit ensures that the level into the modulator will remain constant. The modulator is a variable attenuator that provides vernier level control and AM and compensates for unflatness in any of the following stages. Since there are no filters after the modulator, it must have very good harmonic performance. It is an absorptive modulator that is constructed as two cascaded pi attenuators. The diodes in each leg of the modulator are chosen based on trade-offs between harmonic performance, modulator linearity, and dynamic range. The design evolved from a modulator used in the HP 8663A, with the frequency range extended both higher and lower for use in the HP 8665A. The modulator output drives the high-frequency section of the main output amplifier.